Six different manufacturers' prosthetic heart valves were tested and ranked according to the performance indices described by Swanson and Gabbay. Additionally, a technique was devised whereby the pressure drop across the aortic valve could be predicted by velocity measurements made with continuous-wave Doppler (CWD) imaging. The results of preliminary in vitro studies demonstrate a high correlation (r>0.9) between the peak velocity as measured with CWD and the predicted velocity obtained by the application of a simplified Bernoulli equation, which uses a measured maximum pressure drop across the aortic valve. Also, an acoustical signature analysis technique is under investigation for possible noninvasive detection of partially failed single-leaflet-type mechanical valves.